Lack of TRPV2 impairs thermogenesis in mouse brown adipose tissue

Brown adipose tissue (BAT), a major site for mammalian non‐shivering thermogenesis, could be a target for prevention and treatment of human obesity. Transient receptor potential vanilloid 2 (TRPV2), a Ca²⁺‐permeable non‐selective cation channel, plays vital roles in the regulation of various cellular functions. Here, we show that TRPV2 is expressed in brown adipocytes and that mRNA levels of thermogenic genes are reduced in both cultured brown adipocytes and BAT from TRPV2 knockout (TRPV2KO) mice. The induction of thermogenic genes in response to β‐adrenergic receptor stimulation is also decreased in TRPV2KO brown adipocytes and suppressed by reduced intracellular Ca²⁺ concentrations in wild‐type brown adipocytes. In addition, TRPV2KO mice have more white adipose tissue and larger brown adipocytes and show cold intolerance, and lower BAT temperature increases in response to β‐adrenergic receptor stimulation. Furthermore, TRPV2KO mice have increased body weight and fat upon high‐fat‐diet treatment. Based on these findings, we conclude that TRPV2 has a role in BAT thermogenesis and could be a target for human obesity therapy.     

Ablation of ghrelin receptor reduces adiposity and improves insulin sensitivity during aging by regulating fat metabolism in white and brown adipose tissues

Aging is associated with increased adiposity in white adipose tissues and impaired thermogenesis in brown adipose tissues; both contribute to increased incidences of obesity and type 2 diabetes. Ghrelin is the only known circulating orexigenic hormone that promotes adiposity. In this study, we show that ablation of the ghrelin receptor (growth hormone secretagogue receptor, GHS-R) improves insulin sensitivity during aging. Compared to wild-type (WT) mice, old Ghsr(-/-) mice have reduced fat and preserve a healthier lipid profile. Old Ghsr(-/-) mice also exhibit elevated energy expenditure and resting metabolic rate, yet have similar food intake and locomotor activity. While GHS-R expression in white and brown adipose tissues was below the detectable level in the young mice, GHS-R expression was readily detectable in visceral white fat and interscapular brown fat of the old mice. Gene expression profiles reveal that Ghsr ablation reduced glucose/lipid uptake and lipogenesis in white adipose tissues but increased thermogenic capacity in brown adipose tissues. Ghsr ablation prevents age-associated decline in thermogenic gene expression of uncoupling protein 1 (UCP1). Cell culture studies in brown adipocytes further demonstrate that ghrelin suppresses the expression of adipogenic and thermogenic genes, while GHS-R antagonist abolishes ghrelin's effects and increases UCP1 expression. Hence, GHS-R plays an important role in thermogenic impairment during aging. Ghsr ablation improves aging-associated obesity and insulin resistance by reducing adiposity and increasing thermogenesis. Growth hormone secretagogue receptor antagonists may be a new means of combating obesity by shifting the energy balance from obesogenesis to thermogenesis.     

白藜芦醇抑制小鼠肥胖的功效及相关机制探讨

目的:研究白藜芦醇抑制高脂引起的肥胖的作用机制。方法:将18只C57小鼠随机分为3组,分别为对照组、高脂以及高脂+白藜芦醇小鼠模型,给小鼠喂养一定剂量白藜芦醇(100 mg/kg/d),喂养12周。提取小鼠皮下脂肪细胞,分化成熟,加入白藜芦醇,采用q RT-PCR以及Western blot等方法检测HO-1以及棕色脂肪标志基因的表达。通过q RT-PCR检测小鼠脂肪组织炎症因子、UCP-1以及HO-1的表达。结果:白藜芦醇在体内可以明显抑制高脂引起的肥胖,糖耐量异常,同时促进棕色脂肪标志基因UCP-1,PGC-1以及PRDM16的表达。白藜芦醇还可抑制肥胖小鼠脂肪组织炎症因子的增加以及抗炎蛋白HO-1的表达。在体外分化的成熟的皮下脂肪细胞中,白藜芦醇同样可以促进棕色脂肪标志基因UCP-1,PGC-1以及PRDM16的表达。白藜芦醇通过促进抗炎蛋白HO-1的表达抑制高脂引起的脂肪炎症反应。结论:白藜芦醇可以通过促进白色脂肪棕色化以及抑制慢性低度炎症抑制高脂引起的肥胖、糖耐量异常以及改善胰岛素敏感性。     

UCP1 Induction during Recruitment of Brown Adipocytes in White Adipose Tissue Is Dependent on Cyclooxygenase Activity

Background

The uncoupling protein 1 (UCP1) is a hallmark of brown adipocytes and pivotal for cold- and diet-induced thermogenesis.

Methodology/Principal Findings

Here we report that cyclooxygenase (COX) activity and prostaglandin E₂ (PGE₂) are crucially involved in induction of UCP1 expression in inguinal white adipocytes, but not in classic interscapular brown adipocytes. Cold-induced expression of UCP1 in inguinal white adipocytes was repressed in COX2 knockout (KO) mice and by administration of the COX inhibitor indomethacin in wild-type mice. Indomethacin repressed β-adrenergic induction of UCP1 expression in primary inguinal adipocytes. The use of PGE₂ receptor antagonists implicated EP₄ as a main PGE₂ receptor, and injection of the stable PGE₂ analog (EP_3/4 agonist) 16,16 dm PGE₂ induced UCP1 expression in inguinal white adipose tissue. Inhibition of COX activity attenuated diet-induced UCP1 expression and increased energy efficiency and adipose tissue mass in obesity-resistant mice kept at thermoneutrality.

Conclusions/Significance

Our findings provide evidence that induction of UCP1 expression in white adipose tissue, but not in classic interscapular brown adipose tissue is dependent on cyclooxygenase activity. Our results indicate that cyclooxygenase-dependent induction of UCP1 expression in white adipose tissues is important for diet-induced thermogenesis providing support for a surprising role of COX activity in the control of energy balance and obesity development.     

Raspberry promotes brown and beige adipocyte development in mice fed high-fat diet through activation of AMP-activated protein kinase (AMPK) α1

Development of brown and beige/brite adipocytes increases thermogenesis and helps to reduce obesity and metabolic syndrome. Our previous study suggests that dietary raspberry can ameliorate metabolic syndromes in diet-induced obese mice. Here, we further evaluated the effects of raspberry on energy expenditure and adaptive thermogenesis and determined whether these effects were mediated by AMP-activated protein kinase (AMPK). Mice deficient in the catalytic subunit of AMPKα1 and wild-type (WT) mice were fed a high-fat diet (HFD) or HFD supplemented with 5% raspberry (RAS) for 10 weeks. The thermogenic program and related regulatory factors in adipose tissue were assessed. RAS improved the insulin sensitivity and reduced fat mass in WT mice but not in AMPKα1^-/- mice. In the absence of AMPKα1, RAS failed to increase oxygen consumption and heat production. Consistent with this, the thermogenic gene expression in brown adipose tissue and brown-like adipocyte formation in subcutaneous adipose tissue were not induced by RAS in AMPKα1^-/- mice. In conclusion, AMPKα1 is indispensable for the effects of RAS on brown and beige/brite adipocyte development, and prevention of obesity and metabolic dysfunction.     

Reversine promotes browning of white adipocytes by suppressing miR-133a

Brown adipocytes are characterized by a high number of uncoupling protein 1 (UCP1)-positive mitochondrial content and increased thermogenic capacity. As UCP1-enriched cells can consume lipids by generating heat, browning of white adipocytes is now highlighted as a promising approach for the prevention of obesity and obesity-associated metabolic diseases. Upon cold exposure or β-adrenergic stimuli, downregulation of microRNA-133 (miR-133) elevates the expression levels of PR domain containing 16 (Prdm16), which has been shown to be a brown adipose determination factor, in brown adipose tissue and subcutaneous white adipose tissues (WAT). Here, we show that treatment of reversine to white adipocytes induces browning via suppression of miR-133a. Reversine treatment promoted the expression of brown adipocyte marker genes, such as Prdm16 and UCP1, increasing the mitochondrial content, while decreasing the levels of miR-133a and white adipocyte marker genes. Ectopic expression of miR-133a mimic reversed the browning effects of the reversine treatment. Moreover, intraperitoneal administration of reversine in mice upregulated thermogenesis and resulted in resistance to high-fat diet-mediated weight gain as well as browning of subcutaneous and epididymal WAT. Taken together, we found a novel way to promote browning of white adipocytes through downregulation of miR-133a followed by activation of Prdm16, with a synthetic chemical, reversine.     

Food-derived regulatory factors against obesity and metabolic syndrome

Abstract

Obesity is a key factor in metabolic syndrome. The study of metabolic syndrome focuses on the anti-weight gain properties of physiological mechanisms and food components. Abnormal energy metabolism is a major risk factor of metabolic syndrome. Chronic inflammation is a feature of obesity; cytokines from hypertrophied adipocytes cause inflammation in both adipose tissue and blood vessels, resulting in symptoms of metabolic syndrome. Tumor necrosis factor-α causes insulin resistance in adipocytes and regression of brown adipocytes, resulting in abnormal energy metabolism. Functional foods can serve as a strategy for prevention and treatment of obesity linked with metabolic processes in white and brown adipose tissues. Diet-induced thermogenesis caused by certain food components stimulates burning of stored fat within adipose tissues. A mechanistic understanding of dietary thermogenesis via the sympathetic nerve system will prove valuable for the development of precise strategies for the practical prevention of metabolic syndrome.     

Adipocytes in both brown and white adipose tissue of adult mice are functionally connected via gap junctions: implications for Chagas disease

Adipose tissue serves as a host reservoir for the protozoan Trypanosoma cruzi, the causative organism in Chagas disease. Gap junctions interconnect cells of most tissues, serving to synchronize cell activities including secretion in glandular tissue, and we have previously demonstrated that gap junctions are altered in various tissues and cells infected with T. cruzi. Herein, we examined the gap junction protein connexin 43 (Cx43) expression in infected adipose tissues. Adipose tissue is the largest endocrine organ of the body and is also involved in other physiological functions. In mammals, it is primarily composed of white adipocytes. Although gap junctions are a prominent feature of brown adipocytes, they have not been explored extensively in white adipocytes, especially in the setting of infection. Thus, we examined functional coupling in both white and brown adipocytes in mice. Injection of electrical current or the dye Lucifer Yellow into adipocytes within fat tissue spread to adjacent cells, which was reduced by treatment with agents known to block gap junctions. Moreover, Cx43 was detected in both brown and white fat tissue. At thirty and ninety days post-infection, Cx43 was downregulated in brown adipocytes and upregulated in white adipocytes. Gap junction-mediated intercellular communication likely contributes to hormone secretion and other functions in white adipose tissue and to nonshivering thermogenesis in brown fat, and modulation of the coupling by T. cruzi infection is expected to impact these functions.     

Adipose Rheb deficiency promotes miR-182-5p expression via the cAMP/PPARγ signaling pathway

Dysregulation of microRNAs (miRNAs) in adipocytes plays a critical role in the pathogenesis of obesity. However, the signaling mechanisms regulating miRNAs production in adipose tissue remain largely unclear. Here, we show that adipose tissue-specific knockout of Ras homolog enriched in brain (Rheb), a direct upstream activator of mTOR, increases miR-182-5p level in mouse subcutaneous white adipose tissues. Interestingly, the inhibition of mTOR signaling by rapamycin has no effect on miR-182-5p level in primary subcutaneous white adipocytes, suggesting the presence of a mTOR-independent mechanism regulating Rheb-mediated miR-182-5p expression. Consistent with this view, Rheb-ablation activates the cAMP/PPARγ signaling pathway. In addition, treatment of white adipocytes with pioglitazone, a PPARγ agonist, dramatically upregulates miR-182-5p levels. Our study reveals a unique mechanism by which Rheb regulates miR-182-5p in adipocytes. Given that increasing miR-182-5p in adipose tissue promotes beige fat development, our study also suggests a unique mechanism by which Rheb promotes thermogenesis and energy expenditure.     

Beta 3-adrenoceptor knockout in C57BL/6J mice depresses the occurrence of brown adipocytes in white fat.

White and brown adipocytes are usually located in distinct depots; however, in response to cold, brown adipocytes appear in white fat. This response is mediated by beta-adrenoceptors but there is a controversy about the subtype(s) involved. In the present study, we exposed to cold beta 3-adrenoceptor knockout mice (beta 3KO) on a C57BL/6J genetic background and measured in white adipose tissue the density of multilocular cells and the expression of the brown adipocyte marker uncoupling protein-1 (UCP1). In brown fat of beta 3KO mice, UCP1 expression levels were normal at 24 degrees C as well as after a 10-day cold exposure. Strikingly, under both conditions, in the white fat of beta 3KO mice the levels of UCP1 mRNA and protein as well as the density of multilocular cells were decreased. These results indicate that beta 3-adrenoceptors play a major role in the appearance of brown adipocytes in white fat and suggest that the brown adipocytes present in white fat differ from those in brown fat.     

Follistatin promotes adipocyte differentiation,browning, and energy metabolism

Follistatin (Fst) functions to bind and neutralize the activity of members of the transforming growth factor-β superfamily. Fst has a well-established role in skeletal muscle, but we detected significant Fst expression levels in interscapular brown and subcutaneous white adipose tissue, and further investigated its role in adipocyte biology. Fst expression was induced during adipogenic differentiation of mouse brown preadipocytes and mouse embryonic fibroblasts (MEFs) as well as in cold-induced brown adipose tissue from mice. In differentiated MEFs from Fst KO mice, the induction of brown adipocyte proteins including uncoupling protein 1, PR domain containing 16, and PPAR gamma coactivator-1α was attenuated, but could be rescued by treatment with recombinant FST. Furthermore, Fst enhanced thermogenic gene expression in differentiated mouse brown adipocytes and MEF cultures from both WT and Fst KO groups, suggesting that Fst produced by adipocytes may act in a paracrine manner. Our microarray gene expression profiling of WT and Fst KO MEFs during adipogenic differentiation identified several genes implicated in lipid and energy metabolism that were significantly downregulated in Fst KO MEFs. Furthermore, Fst treatment significantly increases cellular respiration in Fst-deficient cells. Our results implicate a novel role of Fst in the induction of brown adipocyte character and regulation of energy metabolism.     

Uncoupling protein 1 expression and high-fat diets

Uncoupling protein 1 (Ucp1) is the key component of β-adrenergically controlled nonshivering thermogenesis in brown adipocytes. This process combusts stored and nutrient energy as heat. Cold exposure not only activates Ucp1-mediated thermogenesis to maintain normothermia but also results in adaptive thermogenesis, i.e., the recruitment of thermogenic capacity in brown adipose tissue. As a hallmark of adaptive thermogenesis, Ucp1 synthesis is increased proportionally to temperature and duration of exposure. Beyond this classical thermoregulatory function, it has been suggested that Ucp1-mediated thermogenesis can also be employed for metabolic thermogenesis to prevent the development of obesity. Accordingly, in times of excess caloric intake, one may expect a positive regulation of Ucp1. The general impression from an overview of the present literature is, indeed, an increased brown adipose tissue Ucp1 mRNA and protein content after feeding a high-fat diet (HFD) to mice and rats. The reported increases are very variable in magnitude, and the effect size seems to be independent of dietary fat content and duration of the feeding trial. In white adipose tissue depots Ucp1 mRNA is generally downregulated by HFD, indicating a decline in the number of interspersed brown adipocytes.     

京尼平对小鼠棕色和白色脂肪组织UCP1表达的影响

目的:棕色脂肪组织活化和白色脂肪组织棕化是改善减肥的良好策略。本研究利用冷刺激作为阳性对照,观察京尼平对小鼠脂肪组织活化与棕化的作用。方法:8周龄雄性C57BL/6J小鼠30只,随机分为正常对照组、京尼平组、冷刺激组, 每组10只。京尼平组小鼠腹腔注射给予京尼平处理(15 mg/(kg·d),连续9 d),对照组用生理盐水处理,冷刺激组小鼠在室温(22℃±2℃)下处理4 d后,置于4℃环境中进行冷刺激处理5 d(24 h/d)。检测各组小鼠每天摄食量、体重和体温变化,取肩胛下区、腹股沟区及附睾周围部分脂肪组织观察形态学的变化,测定棕色脂肪组织、皮下白色脂肪组织以及内脏白色脂肪组织解偶联蛋白1(UCP1)的表达。结果:与正常对照组相比,京尼平组小鼠白色脂肪湿重下降16%,冷刺激组下降28%,均有明显差异(P＜0.05);京尼平组和冷刺激组白色脂肪组织颜色变深,HE染色显示脂肪细胞内的脂滴变小,数量增加;京尼平组小鼠的皮下、内脏白色脂肪组织和棕色3种脂肪组织中的UCP1表达量均明显增加(P＜0.05)。结论:京尼平通过上调UCP1的表达促进棕色脂肪组织活化和白色脂肪组织棕化,此效应是京尼平降脂减轻体重的作用机制之一。     

Blockade of the activin receptor IIb activates functional brown adipogenesis and thermogenesis by inducing mitochondrial oxidative metabolism

Brown adipose tissue (BAT) is a key tissue for energy expenditure via fat and glucose oxidation for thermogenesis. In this study, we demonstrate that the myostatin/activin receptor IIB (ActRIIB) pathway, which serves as an important negative regulator of muscle growth, is also a negative regulator of brown adipocyte differentiation. In parallel to the anticipated hypertrophy of skeletal muscle, the pharmacological inhibition of ActRIIB in mice, using a neutralizing antibody, increases the amount of BAT without directly affecting white adipose tissue. Mechanistically, inhibition of ActRIIB inhibits Smad3 signaling and activates the expression of myoglobin and PGC-1 coregulators in brown adipocytes. Consequently, ActRIIB blockade in brown adipose tissue enhances mitochondrial function and uncoupled respiration, translating into beneficial functional consequences, including enhanced cold tolerance and increased energy expenditure. Importantly, ActRIIB inhibition enhanced energy expenditure only at ambient temperature or in the cold and not at thermoneutrality, where nonshivering thermogenesis is minimal, strongly suggesting that brown fat activation plays a prominent role in the metabolic actions of ActRIIB inhibition.     

Uncoupling Protein 1 and Sarcolipin Are Required to Maintain Optimal Thermogenesis,and Loss of Both Systems Compromises Survival of Mice under Cold Stress

The importance of brown adipose tissue as a site of nonshivering thermogenesis has been well documented. Emerging studies suggest that skeletal muscle is also an important site of thermogenesis especially when brown adipose tissue function is lacking. We recently showed that sarcolipin (SLN), an uncoupler of the sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA) pump, could contribute to heat production in skeletal muscle. In this study, we sought to understand how loss of UCP1 or SLN is compensated during cold exposure and whether they are both necessary for thermogenesis. Toward this goal, we generated a UCP1;SLN double knock-out (DKO) mouse model and challenged the single and DKO mice to acute and long-term cold exposures. Results from this study show that there is up-regulation of SLN expression in UCP1-KO mice, and loss of SLN is compensated by increased expression of UCP1 and browning of white adipose tissue. We found that the DKO mice were viable when reared at thermoneutrality. When challenged to acute cold, the DKO were extremely cold-sensitive and became hypothermic. Paradoxically, the DKO mice were able to survive gradual cold challenge, but these mice lost significant weight and depleted their fat stores, despite having higher caloric intake. These studies suggest that UCP1 and SLN are required to maintain optimal thermogenesis and that loss of both systems compromises survival of mice under cold stress.